Crop Production under Drought and Heat Stress: Plant Responses and Management Options
Desiccation tolerance and its relationship to assimilate partitioning in winter wheat . Hossain, A.B.S. (USDA, ARS, Kansas State Univ., Manhattan, KS). Desiccation Tolerance and Its Relationship to Assimilate Partitioning in Winter Wheat. A. B. S. Hossain, R. G. Sears,* T. S. Cox, and G. M. Paulsen. ABSTRACT. Desiccation tolerance and its relationship to assimilate partitioning in winter wheat. Cultivars with high grain weights and early release dates were generally.
The biosynthesis of ABA during stress is primarily localised to the vascular tissues and guard cells.
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Molecules such as betaine, proline and glycine are able to reduce water efflux from cells and stabilise cytoplasmic constituents.
Generally, soluble carbohydrate accumulation will occur in tandem with proline and anthocyanin accumulation, aiding the protection against ROS in the first instance.
The induction of a large set of specific proteins are known to reduce and prevent damage to cells at low water contents. LEA proteins are a major representative of protective proteins that are generally hydrophilic, and also have an amino acid composition mostly devoid of cysteine and tryptophan.
In many cases, the function of LEA proteins is not known. It has been suggested that LEA proteins exert their protective role by replacing water to maintain hydration of proteins and other cellular components.
LEA proteins may act in conjunction with carbohydrates to perform this role and may also bind to ions, thereby decreasing ion concentration in dehydrated cells. Biochemical evidence is lacking for these hypotheses. Under normal plant growth conditions, ROS is detoxified using the plant antioxidant defense system.
However, during drought, the balance between aerobic metabolism and ROS detoxification is disrupted. The accumulation of ROS in the cell causes lipid peroxidation, DNA damage and protein carbonylation, triggering a cascade of events ultimately leading to cell death.
This includes but is not limited to the enzymes superoxide dismutase, ascorbate peroxidase and catalase, as well as the reducing compounds ascorbic acid, carotenoids and glutathione. Additionally, the requirements of the agricultural environment are productivity and not just survival.
Targeting carbon for crop yield and drought resilience
Yield penalties in drought tolerant crops are not acceptable. The ideal solution for agriculture is to combine yield and resistance traits together. Much of the yield increase under drought is likely to result from spillover benefits of selection for yield improvement under good growing conditions e.
Delayed silking is a side effect of drought and is commonly used as selection in breeding approaches to drought tolerance for maize.
High stomatal conductance and transpiration seen as low canopy temperature associated with better water uptake are positively correlated with yield under drought and can also be selected for superior performance. This did not result in drought tolerant crops through accumulating trehalose but did initiate a period of enlightenment about the role of T6P instead, now known as a powerful sugar signalling molecule, the modification of which allows significant yield benefits in crops.
The current proliferation of mechanistic understanding, physiological roles and applications of trehalose metabolism in plants and crops began with studies reported in and concerning experiments that heterologously expressed genes for the pathway. First, Escherichia coli genes were expressed in plants with interesting effects on growth and development.
Prior to this time, transgenic plants with altered carbon metabolism were characterised by either minimal phenotypes or strongly negative phenotypes. It appeared that an important mechanism of the regulation of growth and development could be controlled somehow by the trehalose pathway and, given the positive effects, could be utilised in crop improvement. Research over the following 20 years has fulfilled the promise and vision of the early reports with a number of potentially recent significant developments targeting the pathway for crop improvement.
Importantly, this has involved fundamental science in parallel with translation in crops.
Crop Production under Drought and Heat Stress: Plant Responses and Management Options
Studies on heterologous expression in and had shown the potential of the pathway to control growth and development and hence improve crops.
How essential was the pathway? A number of laboratoriess have developed technology to measure micromolar amounts of T6P in plants. Similar observations have been made in other tissues such as wheat grain but where both sucrose and T6P levels are far higher.
T6P relationship still holds strongly. This not only confirmed the sucrose signal hypothesis, but also showed strong tissue and developmental dependency in the regulation of T6P levels. Heat stress could also have major effects on the process of cell divisions Smertenko et al. All these damages can seriously limit the plant growth and also favor the oxidative damage.
In addition to all this brief exposure to the high temperature during the seed filling can result in accelerated filling and will finally result in poor quality and reduction in the yield. Here, we reviewed the basic responses of crop plants to drought and heat stress along with the management options which can be adopted to minimize the harmful effects of these abiotic stresses.
Morphological Responses Growth Drought The initial effect of drought on the plants is the poor germination and impaired seedling establishment. Various studies have reported the negative impacts of drought stress on germination and seedling growth Kaya et al. The reduction in germination potential, early seedling growth, root and shoot dry weight, hypocotyl length, and vegetative growth have been reported in important field crops including, pea Pisum sativum L.
Plant growth is mainly accomplished by cell division, enlargement, and differentiation. Drought impairs mitosis and cell elongation which results in poor growth Hussain et al. Drought limits the process of cell growth mainly due to the loss of turgor Taiz and Zeiger, Water limiting conditions results in impaired cell elongation mainly because of the poor water flow from xylem to the nearby cells Nonami, Number of leaves and the size of individual leaf are also reduced under the drought conditions.
The expansion of the leaf normally depends upon the turgor pressure and the supply of assimilates. Reduced turgor pressure and slow rate of photosynthesis under drought conditions mainly limit the leaf expansion Rucker et al. Fresh and dry weights are also severely reduced under the water limiting conditions Zhao et al. Plant height, leaf size, and the stem girth were significantly reduced under the water limiting conditions in maize Khan et al.
In another study, Kamara et al. Heat Stress Under the tropical climates the excessive radiations and elevated temperatures are another major limiting factor to plant growth and development. High temperatures may cause scorching of the twigs and leaves along with visual symptoms of sunburn, leaves senescence, growth inhibition and discoloration of fruits and leaves Ismail and Hall, ; Vollenweider and Gunthardt-Goerg, Elevated temperatures can reduce the germination potential of the seeds and, thus, results in poor germination and stand establishment.
Adverse effects of high temperature on cereal crops vary with the timing, duration, and sternness of the heat stress Fahad et al. High temperature stress reduced number of spikes and number of florets per plant in rice and seed-set in sorghum was also negatively affected under similar conditions Prasad et al. Inside a floret, anthers, and pollens were more susceptible to high temperature than ovules. A significant reduction in the growth and net assimilation rate was observed in maize and sugarcane Saccharum officinarum L.
Yield Drought Yield is basically the complex integration of the different physiological processes. Most of these physiological processes are negatively affected by the drought stress. The negative impacts of drought on the yield mainly depend upon the severity of the stress and the plant growth stage. The drought induced at the pre-anthesis stage shortened the time to anthesis while that applied after anthesis reduced the period of grain filling in cereals Estrada-Campuzano et al.
The process of the grain filling in cereals is controlled by four major enzymes, i. A decreased activity of these enzymes has been reported under the drought conditions which have a negative impact on the yield of major cereal Ahmadi and Baker, The exposure of plants to drought stress at the flowering may result in complete sterility in pearl millet Pennisetum glaucum L. Table 1 Yield losses in some major crops caused by drought and heat stress.